The prior art shows an electronically controllable implantable drug delivery system in which a pump is based on an electro-osmotic principle, since such a motive arrangement could readily be controlled by electronic means, either in response to internal pressure sensors, or by direct actuation by externally induced signals. It will be noted that the prior art does not teach closed loop blood serum sensing; for example blood sugar level, and dispensing of medication to maintain the desired level. The prior art and the present invention are concerned only with faultless medication by dispensing prescribed dosages of drugs, for example, Insulin, according to a prescribed regimen controllable from outside the body in which the drug dispensing unit is implanted, or alternatively, the dispensing of drugs automatically on a monthly basis under the control of long-term timing electronics. The automated system would preferably have an override capability for unscheduled dosage adjustment. For example, diabetic patients often know at the onset of symptoms of hyperglycemia, from long experience with the disease, when, and how much Insulin should be injected. and are capable of operating an implanted Insulin dispensing system manually. The implanted reservoir and electrically operated dispenser saves the diabetic patient the discomfort of periodic, often daily, injections, in favor of refilling the reservoir only once per month, or longer.
The closest prior art described in the literature contemplates the use of an implantable container similar in volume to that of a cardiac pacemaker, having a biocompatible material or coating at the interface with living tissue. The container is divided into three chambers, as shown in FIG. 1, illustrating the prior art. The rightmost chamber 11 is a reservoir which contains the Insulin fluid in quantity sufficient for delivery over an appreciable period; e.g., 1 cc per day for 1 month before replenishment is needed. The exit from this reservoir is connected with an internal conduit 12 of the body, such as an artery, through a tube 13 having a permeable end 13A. Immediately adjacent to this reservoir 11 and separated therefrom by a flexible impermeable diaphragm 14 is a second chamber 15 which contains a drive fluid such as water. This chamber is initially filled without causing deflection of the diaphragm 14 and thus, at rest, does not exert any pressure on the dispensable fluid in the reservoir 11.
When pressure is exerted on the flexible diaphragm 14, Insulin is forced out of the reservoir and into the artery. The pressure is produced by an electro osmotic force. The third chamber 16, to the left of the drive chamber 15, also contains drive fluid (water). The two chambers are separated by a rigid wall 17 which includes a centrally disposed permeable membrane 17A. A source of electrical potential such as a battery, is connected between the two chambers 15, 16. When current is caused to flow, electro-osmotic migration of fluid occurs between the chambers through the membrane 17A. When the current is of one polarity, fluid flows from the leftmost chamber 16 to the center chamber 15, and when the current is of the other polarity, fluid flows in the opposite direction. When flow is toward the flexible diaphragm 14, the compressive force caused thereby reduces the volume of the drug reservoir 11 and Insulin is driven by the resulting pressure into the artery against the internal pressure of the blood flow in the artery. A check valve (not shown) prevents back-flow of blood into the device when the pressure of the blood flow exceeds the pressure in the drug dispensing reservoir.
It might seem that all the drug in the reservoir could be dispensed into the artery as current is applied to transfer the drive fluid from the leftmost chamber 16 to the center chamber 15. However, there is a limitation on that pumping capability of the prior art system. To appreciate this, reference is directed specifically to the ullage space designated as A, above the level of the fluid in the leftmost chamber 16. It is known that in order to pump a fluid from one chamber to another through an interconnecting valve, (e.g., the permeable membrane 17A) the pressure in one chamber must be greater than the pressure in the other. It should be noted that the permeable membrane 17A is directional one way valve dependent on the electrical polarity. Obviously, if the pressures are equal, no flow can take place because the opposing pressures cancel. Thus, if the fluid level in the leftmost chamber 16 is the same as that in the center chamber 15, that is, both are completely filled, and if current is caused to move fluid from the left to the right, as water moves to the right, a vacuum would be formed in the left chamber at A. The resultant pressure differential would produce water back-flow which would continuously return the water to the leftmost chamber. If however, gas fills space A, transfer of water from the left to the right under electro-osmotic impulse could take place, but only up to a limit, which is of practical importance. As water is driven from the left to the right, through the permeable membrane 17A, the space A becomes enlarged. Gas in the space decreases in pressure as a large volume is filled by the confined gas. Thus, when the volume of space A reaches a critical dimension, the pressure differential between the two fluid volumes in chambers 15 and 16 will reach the maximum capability of the pump, and pumping will cease even though current is still being applied. The effect of this is that from the point of view of volume, in order to dispense a given amount of drug, the container would have to be physically much larger than is desirable because the reservoir 11 does not completely empty.